1. Field of the Invention
The present invention relates to a lead-free and preferably arsenic-free optical dense crown glass, to the use of a glass of this type for the fields of imaging, projection, telecommunications, optical communication technology and laser technology, and to optical elements and preforms of these optical elements.
2. Description of the Related Art
In recent years, in both optical and opto-electronic technologies (application areas including imaging, projection, telecommunications, optical communication technology and laser technology), there has been a considerable market trend towards miniaturization. This is apparent from the ever-decreasing size of end products and of course requires increasing miniaturization of the individual modules and components of end projects of this type. For producers of optical glasses, this trend, despite increasing numbers of end products, entails a considerable drop in volume demand for raw glass. At the same time, the further processors are putting increasing pressure on glass manufacturers' prices, since the production of smaller components of this type from block glass and/or draw rod glass in percentage terms, based on the product, entails significantly increased scrap, and the, machining of extremely small parts of this nature entails higher costs than larger components.
As an alternative to separating optical components out of block glass or draw rod glass, as has hitherto been customary, production processes in which performs, which are as near net shape as possible, can be obtained directly from the glass melt have recently been becoming increasingly important. For example, the further processors are demanding precisely pressed small components or precursors thereof, i.e. near net shape preforms, for repressing, known as precision gobs. Precision gobs are generally understood as meaning preferably completely fire polished, semi-free or free formed glass portions, which are accessible via various production processes.
One method for producing precision gobs is the bead spraying process, in which glass beads with a defined size distribution are produced from the glass melt by means of a spraying process. The desired size fraction(s) is/are separated off, for example, by screening. The residual fraction does not have to be discarded, but rather can be recycled as high-purity cullet, which can be melted down again particularly well. This process, which is very simple to carry out in terms of technology and labor requirement and does not require targeted portioning of the glass strand, allows large numbers to be produced within a short time.
However, the process of near net shape direct pressing, which is higher up in the value-added chain, known as blank pressing, is more advantageous. This process allows the smaller volumes of glass melt (distributed over a large number of small pieces of material) to be flexibly counteracted by means of short set-up times. However, compared to gob spraying, the reduced number of cycles and pieces and, with small geometry, the added value cannot be derived from the value of the material alone. Therefore, the products have to leave the press in a state, which is “ready for system installation”, i.e. it must be possible to dispense with the expensive re-machining, cooling and/or further processing in the cold state. On account of the highly accurate geometry required, precision equipment with high-quality and therefore expensive mold materials must be used for a pressing process of this type. The service lives of these molds are a huge factor in the profitability of the products and/or materials produced. An extremely important factor with regard to achieving a long service life is that the operating temperature must be as low as possible but this operating temperature can only be reduced to an extent that still ensures that the viscosity of the materials, which are to be pressed, remains sufficient for the pressing operation. Therefore, there is a direct causal link between the processing temperature and therefore the transformation temperature Tg of a glass which is to be processed and the profitability of a pressing operation of this type: the lower the transformation temperature of the glass, the longer the mold service lives become and the greater the profit margin. This relationship therefore results in the need for what are known as “low-Tg glasses”, that is to say glasses with low melting and transformation points, i.e. glasses, which have a viscosity, which is sufficient for them to be worked at the lowest possible temperatures.
Recently, an increased demand for “short” glasses has been reported as a further customer criterion with a view to the melt process technology, that is to say a demand for glasses whose viscosity varies considerably with a relatively slight change in the temperature. In the melting process, this behavior has the advantage that the hot-forming times, i.e. the mold closure times, can be reduced. This firstly increases the throughput, i.e. reduces the cycle time, and secondly is gentler on the mold material, which, as has been described above, also has a positive effect on the overall production costs. Short glasses of this type have the further advantage that on account of the more rapid cooling than with corresponding longer glasses it is also possible for glasses with a stronger tendency to crystallize to be worked. This avoids the need for preliminary nucleation, which could cause problems in subsequent secondary hot-forming steps. This opens up the possibility of also enabling glasses of this type to be drawn into fibers.
Although the prior art has already described glasses with a similar optical position or similar chemical composition, these glasses have significant drawbacks.
EP 0 566 866 (Corning) relates to ophthalmic zinc phosphate glasses with an obligatory addition of Ag2O and Tl2O3 amounting to in total at least 4% by weight. In this case, these two extremely expensive components are required for ion exchange. In addition, they make the melting process far more difficult, on account of the redox sensitivity of Ag(Ag(1) to Ag(0) and the considerable toxicity of Tl2O3.
U.S. Pat. No. 5,021,366 (Corning) describes ophthalmic zinc phosphate glasses with an obligatory Li2O content of at least 1% by weight, which in that case is needed in order to set the specific optical position. However, the addition of Li2O is disadvantageous, since this component is relatively expensive and increases the tendency to crystallization.
EP 0 494 358 (Corning) relates to low-melting industrial zinc phosphate glasses. However, these glasses also include a chlorine content, which can be detected by analysis and has an adverse effect from the internal optical quality of the glasses.
JP 02-124743 relates to optical glasses which can be used for blank-pressing and which have a similar composition to the glass according to the invention.
However, the glass composition described in this document does not contain any sodium oxide.
The glasses disclosed in JP 11-268927 (Hoya) contain CuO, which imparts a strong color and is therefore strictly forbidden for conventional optical applications/glasses, as an obligatory component in an amount of up to 10% by weight. An obligatory CuO content of this nature (up to 12% by weight) is also described in JP 09 100 136 (Hoya).
The types of glass disclosed in JP 63-021240, wit their obligatory Nd2O3 content (up to 33% by weight), constitute active laser glasses. An activity of this nature is ruled out for conventional optical applications on account of the undesirable beam modulation, which is to be expected.
The glasses described in U.S. Pat. No. 4,871,230 (Hoya) are not zinc phosphate glasses, but rather alumino-phosphate types, the viscosity-temperature characteristics of which are not comparable to those of zinc phosphate glasses. Their melting point is generally significantly higher and therefore they cannot be referred to as low-Tg glasses. Furthermore, even when La2O3 is added to glasses of this nature, the required shortness for precision gobs or moldings cannot be achieved.
JP 61-040839 (Ohara) describes an extremely cord-free optical glass, but this property has to be ensured, inter alia, by the obligatory, extremely high refining agent content (Sb2O3≧1% by weight). However, such a high level of added Sb2O3 is not possible for the above-mentioned modern optical application areas, on account of the expected deterioration in transmission caused by the intrinsic absorption of this component.